With the increase of operating speed of central processor units in the electronic computer systems, further speed-up of the high-speed printing apparatus, which is an output unit, has been required and various improvements have been made.
A typical example of such improvements is a system in which ink is supplied to the nozzle under a very low pressure, such that the ink will assume a hemispherically projecting state at the nozzle end and an electric field is applied between the nozzle and an accelerating electrode positioned several millimeters in front of the nozzle to draw out the ink in the form of droplets, and then a strong electric field is provided between the nozzle and the platen to let the ink droplets move toward the surface of the printing sheet, so that said ink droplets are electrostatically deflected in both main and auxiliary directions (right and left) to thereby print letters, signs or such on the surface of the printing sheet.
There is also known a system in which the ions produced between the electrodes which receive a high voltage are passed through the ink mist so that they are deposited with the ink particles, and such ion-carrying ink particles are adsorbed on the surface of the printing sheet.
The present invention concerns a further improvement in the latter system, so this system is first described in further detail for a better understanding of the present invention.
Referring to FIG. 1, when a high voltage on the order of several thousand volts is applied between the anode 101 formed from a wire electrode and the cathode 103 in the platen 102, cations are produced from said anode 101. These cations tend to migrate toward the cathode by passing the aperture 104a in each aperture board 104, but such migration is retarded or promoted depending on the potential of the selective electrode 104b. That is, an ion controlling electrode comprising a common electrode 104c arranged to always maintain a constant voltage relative to said selective electrode 104b and a insulating member 104d interposed between said two electrodes is formed centering around the aperture 104a, and the selective electrode 104b is connected to a drive circuit (not shown), so that it is selectively driven to a positive or negative potential relative to the constant potential of the common electrode 104c. Therefore, if the potential of the selective electrode 104b is higher than that of the common electrode 104c, the electric field in the aperture 104a is orientated in the direction of the cathode and hence the cations can pass through said aperture 104a, but if the potential of the selective electrode 104b is lower than that of the common electrode 104c, the electric field in the aperture 104a is directed in the opposite direction and hence passage of the cations is prevented. Thus, in order to obtain a desired printing pattern, all the selective electrodes 104b are driven to control said movement of the cations for all of the aperture 104a. The cations which have passed the aperture 104a enter the ink mist 105, and while passing through this ink mist, they are deposited with fine ink particles and further continue their movement toward the cathode 103 until they are adsorbed on the surface of a printing sheet 106. Thus, dots are printed on the printing sheet 106 by the ink particles carried on the cations. This is effectuated throughout the entire width of the printing sheet 106, and hence the letters (figures, etc.) formed from plural dot patterns are printed all together in lines, with the above-said operation being programmed as main scanning and feed of the printing sheet 106 as auxiliary scanning.
The ink mist 105 is produced from the ink solution 108 in a mist tank 107 when vibrators 109 made of a piezoelectric material are vibrated by a high-frequency electric pulse, and such ink mist is supplied to the printing section. There exist air streams on both upper and lower sides of said ink mist 105. These air streams serve to prevent the printing sheet 106 and aperture board 104 from being soiled by said ink mist 105. For this reason, the ink mist 105 flows in the form of a strip as shown in the drawing. The circulation system of these ink mist and air streams consists of a separator 110 adapted for collecting the ink particles alone from the mixture of the ink particles and air, a cooler 111 for cooling air, a main pump 112 for feeding air into the mist tank 107 and printing section, and air pipes connecting said means.
FIG. 2 is a perspective view of the printing section in the above-described type of high-speed printing apparatus, and FIG. 3 is a top plane view of an aperture board 104. In the surface of said aperture board 104 are formed a plurality of apertures 104a arranged in staggered relation to each other and along the length of the aperture board 104 as shown, and an ion controlling electrode is provided for each of said apertures 104a.
One of the serious problems encountered in the above-described type of high-speed printing apparatus is that the ink mist density must be maintained uniform throughout the full width of the printing sheet. FIG. 4 shows a horizontal section of the mist tank 107 horizontally cut along the central portion thereof. The lateral length of this tank corresponds to the width of the printing sheet, and a plurality of vibrators 109 are provided at a given interval at the bottom of the tank. If even one of these vibrators should be damaged, the ink mist density at the printing position corresponding to the damaged vibrator is reduced and consequently the printed letters are blurred.